Fracture properties of interfacially doped Nb-A12O3 bicrystals: I,fracture characteristics

The influence of orientation and impurities on the fracture behavior of Nb–sapphire interfaces was studied using notched bending tests. Single crystals were diffusion bonded in UHV for different interface orientations. The bicrystals were doped to produce prescribed fractional interfacial coverages of Ag. The interfacial impurity content was measured after fracture with Auger spectroscopy.The tougher bicrystals exhibit significant nonlinearity in loading. A J-integral analysis was used to account for the large plastic zones. For undoped bicrystals bonded at 1400°C, the interfacial fracture energy ranged from J_c of 77 to 2100 J/m² depending on the interface planes of the Nb and sapphire. Greater toughnesses were derived from bonding at 1300°C, owing to less oxygen contamination of the Nb. Interfacial doping by Ag atoms leads to a strong reduction of J_c at coverages of only 0.2 to 0.5 of a monolayer. Higher fracture energy is caused by greater plastic deformation in the Nb as observed by slip lines on the metal fracture surface. Evaluation of the loading and fracture characteristics revealed that sharp precursor cracks developed initially in the ceramic. Extensive crack blunting also occurs, especially for the tougher bicrystals, but is often followed by erratic or unstable extension during which far less plasticity occurs, apparently owing to the rate sensitivity for Nb deformation.     

Effect of Heat Treatment, Pre-stress and Surface Hardening on Fracture Toughness of Micro-Alloyed Steel

Micro-alloyed steels are being increasingly accepted by industry in various fields of application and are available with a wide variety of microstructures. Extensive literature is available on their microstructure-property relationships. The superior mechanical properties of micro-alloyed steels are caused by fine-grained microstructures and precipitation of micro-alloying elements such as V, Ti and Nb that led to an improvement in yield strength, in the product of tensile strength and total elongation and in Charpy V-notch impact energy as well. The microstructural changes caused by heat treatment or residual stress state caused by surface hardening or mechanical means may influence the fracture toughness of these micro-alloyed steels. It is in this context that the present work begins with experimental determination of quasi-static initiation fracture toughness (J _1c) of low carbon (0.19%) micro-alloyed steel in as-rolled condition without any heat treatment. The study further explores the effect of normalizing, shot-peening and cyaniding followed by shot-peening on fracture toughness of as-rolled steel under study. The normalizing heat treatment, shot-peening and cyaniding followed by shot-peening—each indicates a positive influence on initiation fracture toughness. Results, when compared, show that cyaniding followed by shot-peening have led to a 2.7 times increase in J _1c. Cyaniding followed by shot-peening may therefore be considered as having the most positive influence on initiation fracture toughness in as-rolled condition for the type of micro-alloyed steel under study. Although initiation fracture toughness is in general known to decrease with increase in yield strength in LEFM arena, the micro-alloyed steel under study when normalized displayed simultaneous improvement in yield strength and J _1c. All these observed effects of normalizing, shot-peening and cyaniding on initiation fracture toughness (elastic-plastic fracture mechanics) were explained on the basis of microstructural study and stress depth profiles.     

活性元素Ti和SiO2界面结合机制的第一性原理计算

Ti元素是钎焊SiO_2f/SiO₂复合材料重要的活性元素,因此,使用第一性原理计算研究了Ti和SiO₂的界面结合机制. 分别建立了两种不同的终止面和化学计量比的界面,使用界面分离功、电子行为和界面能研究了界面原子间的结合. 结果表明,在O终止界面中,界面处Ti和O形成很强的离子-共价键,界面分离功最大可达到8.99 J/m2. 在Si终止面界面中,Ti和Si形成共价-离子键,界面分离功为2.65 J/m2. 在温度为1 173 K时,当Si的活度大于e?35时,富Si界面的界面能更低,界面倾向于形成Ti-Si化合物. 当Si的活度小于e?35时,富O界面在热力学上更加稳定,界面倾向于形成Ti-O化合物. SiO₂中的Si被Ti置换出后,Si扩散进入钎料,活度升高,与钎料中的Ti反应生成Ti-Si化合物,所以界面结构为SiO₂/Ti-O化合物/Ti-Si化合物/钎料.     

Infrared joining strength and interfacial microstructures of Ti–48Al–2Nb–2Cr intermetallics using Ti–15Cu–15Ni foil

Infrared joining of Ti–48Al–2Nb–2Cr using Ti–15Cu–15Ni (wt%) foil as brazing filler metal was investigated at the temperature range of 1100∼1200°C for 30∼60 s in a flowing argon environment. The compressive tests show three types of fracture morphologies in which type I fails at the joint interface, but types II and III are fractured in the base-metal with the crack direction parallel to and perpendicular to the loading axis, respectively. Most of joined specimens were fractured through the base metal indicating that the infrared joined interface has relatively good joint strength. The compressive strength of type I specimen is about 319–322 MPa. Experimental results show that the shorter the real holding time or the higher the joining temperature, the larger the strength variation will be. The observed interfacial microstructures of Ti–48Al–2Nb–2Cr joint interfaces indicate that seven characteristic zones can be distinguished in the joint interfaces and each characteristic structure corresponds to one or more stable phases at T_w temperature. The observed microstructures and their evolutions of each zone are explained in detail in this study. The major difference between joint interfaces of Ti–48Al–2Nb–2Cr and Ti₅₀Al₅₀ alloys takes place on the base-metal interface zone and the columnar two-phase zone. The existence of Nb and Cr atoms in Ti–48Al–2Nb–2Cr alloy also has some influences on the microstructural evolution of the columnar two-phase zone and the continuous α₂-layer.     

高密度脉冲电流处理对Ti2AlNb基合金板材的增塑作用

为了优化Ti-22Al-27Nb基合金板材的塑性,研究了高密度脉冲电流(Jmax=6.80～7.09 kA/mm2,tp=110 μs)处理对Ti-22Al-27Nb合金板材力学性能和组织的影响.应用扫描电子显微镜(SEM)观察了试样的微观组织和形貌变化,应用单向拉伸试验对不同状态试样的力学性能进行了测试.结果表明,高密度脉冲电流处理能够细化Ti-22Al-27Nb合金板材的晶粒,促进塑性的提高.具有细小而均匀显微组织的试样表现出最好的塑性,其伸长率可达到19.4％.对晶粒细化的机制进行了分析,相变过程中形核率的加快和极短的脉冲电流处理时间是晶粒细化的主要原因.     

Structural stability of characteristic interface for NiTi/Nb Nanowire: First-Principle study

Compared with some other conventional interface models, the interface of NiTi(211)/Nb(220) in NiTiNb metal nanocomposite had been simulated and analyzed carefully. Results show that only several interface models, i.e., NiTi(100)/Nb(100)(Ni?Nb), NiTi(110)/Nb(110) and NiTi(211)/Nb(220), can be formed accordingly with their negative formation enthalpy. Therein the cohesive energy ΔE and Griffith rupture work W of NiTi(211)/Nb(220) interface model are the lowest among them. Density of states shows that there exists only one electronic bonding peak for NiTi(211)/Nb(220) interface model at -2.5 eV. Electron density difference of NiTi(211)/ Nb(220) shows that the Nb-Nb, Nb-Ti and Nb-Ni bonding characters seem like so peaceful as a fabric twisting every atom, which is different from conventional metallic bonding performance. Such appearance can be deduced that the metallic bonding between Nb-Nb, Nb-Ti and Nb-Ni in NiTi(211)/Nb(220) may be affected by its nanostructure called nanometer size effect. Thus, our findings open an avenue for detailed and comprehensive studies of nanocomposite.     

First-principles study of influence of Ti vacancy and Nb dopant on the bonding of TiAl/TiO2 interface

Influence of defects (Ti vacancy and Nb dopant) on the bonding of TiAl/TiO₂ interface was studied via first-principles calculations. It was shown that the bonding strength and the stability of TiAl/TiO₂ interface were weakened by the presence of Ti vacancy and dopant Nb. The defects could also change the relative stability of the interface with different couplings between the two compounds. Electronic structure of the interface was analyzed and the influence mechanisms of defects on the bonding of interface were presented.     

First-principles study of the ideal cleavage fracture of Cr2Nb microalloyed by X (Al,Ni, Co,Ti)

The structural properties and elastic constants of the pure Cr₂Nb, the ideal cleavage fracture properties of the Cr₂Nb with the elements X (Al, Ni, Co and Ti) and site preference of the elements X were investigated using the first-principles method. Our calculated lattice parameters and elastic constants of the pure Cr₂Nb are in agreement with the experimental data. Moreover, our calculated results show that the elements X (Al, Ni and Co) occupy the Cr site, and Ti the Nb site in Cr₂Nb. The cleavage energies G_c and critical cleavage stress σ_c with and without the additive elements in the cleavage planes were calculated. The results demonstrate that the elements X (Al, Ni, Co and Ti) could be used to improve the cleavage properties of Cr₂Nb. Finally, the electronic mechanism behind the effects of the additive elements on the ideal cleavage properties of Cr₂Nb was investigated by calculating the electronic structure.     

Experimental determination of interfacial toughness curves

The evaluation of mixed-mode interfacial fracture toughness is of great importance for the integrity of electronic devices because interfacial cracks are subjected to mixed-mode loading by vapor pressure, thermal stress and other factors. A lot of work has been done to clarify the unusual characteristics of the fracture of bimaterial interfaces. Since interfacial fracture toughness (G _c) greatly depends on phase angle (Ψ), most work has focused on the determination ofG _c-Ψ curves using various specimens. However, this requires a lot of time, expense and effort. In this work, theG _c-Ψ curves of oxidized Cu-based leadframe/EMC (epoxy molding compound) interfaces were determined by measuring interfacial fracture toughness under mode-I, mode-II and mixed-mode loading conditions using sandwiched-double-cantilever-beam (SDCB) specimens, Arcan specimens and sandwiched-Brazilian-nut (SBN) specimens, respectively. The experimental results showed that the measuredG _c-Ψ curves obeyed theG _c-Ψ expression proposed by Ahmad first developed by Evans and Hutchinson based on the micromechanics modeling of mixed-mode fracture. Comparing data in the literature, theG _c-Ψ expression proposed by Ahmad was confirmed to be valid, and a method of predictingG _c-Ψ curves without much experiment was proposed.     

Behaviors of interfacial elements during the brazing of SiO2 glass ceramic to Ti-6Al-4V with AgCu/Ni interlayer

AgCu/Ni composite interlayer was used to join SiO₂ glass ceramic to Ti-6Al-4V alloy successfully, obtaining the largest joint shear strength 110MPa. Ag, Cu and Ni in the interlayer and Ti in the Ti-6Al-4V alloy affect the joint formation and interfacial products significantly. To understand the joint formation process better, behaviors of elements Ag, Cu, Ni and Ti during the brazing of SiO₂ glass ceramic to Ti-6Al-4V alloy were investigated in the present work. Active element Ti is the most important component in the joining, realizing the metallurgical bonding of SiO₂ glass ceramic to braze alloy. Cu together with Ni reacts to Ti in the base material by Ti-Cu-Ni ternary eutectic reaction, which is beneficial for reducing the massive Ti-Cu and/or Ti-Ni brittle intermetallic compounds on the joint interface. Dispersion of Ag decreases the brittleness of the whole joint effectively.     

Atomic-scale structure and chemistry of ceramic/metal interfaces—II. Solute segregation at MgO/Cu (Ag) and CdO/Ag (Au) interfaces

The first quantitative measurements of solute segregation at ceramic/metal (C/M) heterophase interfaces are presented for the MgO/Cu (Ag) and CdO/Ag (Au) systems. Interfaces are produced by internal oxidation of ternary alloys. Solute segregation at C/M interfaces is induced by intermediate-temperature annealing treatments. The Gibbsian interfacial excess of solute, Γ_solute, at these interfaces is determined in a direct, quantitative manner by atom-probe field-ion microscopy (APFIM). These measurements are complemented in the MgO/Cu (Ag) system by a composition analysis of this interface employing electron energy loss spectroscopy (EELS). Analyses of 15 {222} MgO/Cu (Ag) interfaces by APFIM show an average segregation level of (4.0±1.9)×10¹⁴ atoms/cm² or 0.22±0.10 effective monolayers at 500°C. Analyses of three {222} CdO/Ag (Au) interfaces show an average segregation level of (3.0±1.0)×10¹⁴ atoms/cm² or 0.22±0.07 effective monolayers at 400°C. Whereas {222} CdO/Ag (Au) interfaces in unannealed specimens show no evidence of gold segregation. These results are discussed in view of recent models of interfacial segregation.     

A first-principles study on interfacial properties of Ni(001)/Ni3Nb(001)

The Ni (001) surface, Ni₃Nb (001) surface and Ni (001)/Ni₃Nb (001) interfaces were studied using the first-principles pseudopotential plane-wave method. The adhesion work, thermal stability and electronic structure of Ni/Ni₃Nb (001) interfaces were calculated to expound the influence of atom termination and stacking sequence on the interface strength and stability. Simulated results indicate that Ni and Ni₃Nb (001) surface models with more than eight atomic layers exhibit bulk-like interior. The (Ni+Nb)-terminated interface with hollow site stacking has the largest cohesive strength and critical stress for crack propagation and the best thermal stability among the four models. This interfacial Ni and the first nearest neighbor Nb atoms form covalent bonds across the interface region, which are mainly contributed by Nb 4d and Ni 3d valence electrons. By comparison, the thermal stability of Ni/Ni₃Nb (001) interfaces is worse than Ni/Ni₃Al (001) interface, implying that the former is harder to form. But the Ni/Ni₃Nb interface can improve the mechanical properties of Ni-based superalloys.     

First-Principles Calculation of Nb2AlC/Nb Interfaces

We have performed a first-principles density functional theory method and molecular dynamics simulation on the Nb₂AlC(001)/Nb(001), Nb₂AlC(001)/Nb(110), and Nb₂AlC(001)/Nb(111) interfaces. The results show that the Nb₂AlC(001)/Nb(111) interface structure is the most stable structure of the three. The Al-Nb bonds at the Nb₂AlC(001)/Nb(111) interfaces show covalence bonding character, while the Nb-Nb interface bonds are mainly metallic. The Nb-C bonds in Nb₂AlC layers are very stable at up to 1500 K temperature and in an oxygen environment. The stable Nb₂AlC(001)/Nb(111) structure may have very good oxidation resistance for applications in high-temperature turbines.     

The connection between ab initio calculations and interface adhesion measurements on metal/oxide systems: Ni/Al2O3 and Cu/Al2O3

The Ni/Al₂O₃ and Cu/Al₂O₃ interfaces have been examined by atomistic, first-principles computations. Relationships have been established with such metallurgical variables as the activity of aluminum and the partial pressure of oxygen. The calculations reveal that the interfaces could be either stoichiometric, or Al-rich, or O-rich, depending on the Al activity. The results are amenable to comparison with available sessile drop and fracture measurements. For conditions applicable to sessile drop experiments performed with Ni(Al) or Cu(Al), the calculations reveal that, as the Al activity increases, initially the work of adhesion increases, reaches a maximum, and finally decreases to that for pure Al. This trend is consistent with the known measurements. Interfaces generated by diffusion-bonding with ‘pure’ Ni or Cu are predicted to be O-rich, with a large work of separation, W_sep. The implication is that the separation process induces substantial plastic dissipation in the metal, consistent with the high interface toughness. For interfaces formed through Al₂O₃ growth on Ni(Al) alloys, the interface is predicted to be Al terminated, with W_sep several times smaller than for either bulk Ni or Al₂O₃. This reduction is in accordance with observations that these interfaces fail in a brittle manner with no noticeable plasticity.     

Mo对NbCr2/ Nb合金组织及性能影响

采用机械合金化+热压工艺制备了NbCr_2/Nb-XMo (X=0,2.5,5.0,7.5,10, at%)合金,研究了合金元素Mo对NbCr_2/Nb合金组织及性能的影响。结果表明:合金元素Mo主要存在于Nb基体中,对合金的物相不产生明显影响,合金仍由Nb固溶体和NbCr_2组成;Mo的添加使得NbCr_2/Nb的相界面处应力增加,导致NbCr_2颗粒中的层错/孪晶的密度增加,并促进了Nb基体中位错的运动,从而使得NbCr_2/Nb合金在保持高强度的同时,具有良好的塑性和韧性。     

钛/铝/钛层状复合材料的爆炸复合制备及后续热压处理研究

本文以纯钛板与纯铝板为原料,通过爆炸复合法制备钛/铝/钛层状复合材料,之后采用热处理以及热压工艺对钛/铝/钛层状复合材料进行进一步处理。研究结果表明：复合板界面主要由波状界面和平直状界面构成,铝元素与钛元素在界面上发生了互扩散,界面结合性能优良,可以承受后续较大的二次塑性变形;热处理后的复合板界面发生明显扩散,在热处理25 h后热压2.5 h后铝层完全反应,扩散反应层主要由TiAl₃相以及Ti₂Al₅相构成。     

Effects of test orientation on fracture and fatigue crack growth behavior of third generation as-cast Ti–48Al–2Nb–2Cr

The effects of changes in test orientation and load ratio on the room temperature fracture and fatigue crack growth behavior of as-cast Ti–48Al–2Nb–2Cr titanium aluminide was investigated to determine the presence of any anisotropy in mechanical properties. As-cast samples were tested in the longitudinal and transverse directions to the casting direction at room temperature in air. Load ratios ranging from R = 0.1 to R = 0.9 were used in the fatigue tests in order to determine its effects on the threshold for fatigue cracking, the Paris law slope, and fatigue crack instability toughness, K_c, in addition to determining both notched and fatigue-precracked values for toughness. Optical metallography and SEM fractography were used to document the effects of orientation on the fracture path and morphology. Significant effects of changes in load ratio were obtained on the fatigue threshold and Paris law slope, while its effects on K_c and the effects of sample orientation were found to be minimal. These are rationalized by considering microstructural effects on the properties measured and are compared to similar materials processed via different techniques.     

Effect of Ti,Nb, and Ti + Nb Coatings on the Bond Strength-Structure Relationship in Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Joints

There is a growing interest in metal-ceramic bonding for wide range of applications in electronic devices and high technology industry for fabrication of metal matrix composites and bonding of ceramic components to metals. The object of the work was to study the effect of Ti, Nb, and Ti + Nb thin films deposited by PVD method on alumina substrates on structure and bond strength properties of Al/Al₂O₃ joints. The joints were fabricated using the results of a wetting experiment and the sessile drop method at a temperature of 1223 K in a vacuum of 0.2 MPa for 30 min of contact. The structure of the metal/ceramic interface was investigated using scanning electron microscopy. The elemental distribution at the metal-ceramic interface was analyzed using energy dispersive x-ray spectroscopy. Transmission electron microscopy was also used to investigate some aspects of the metal/ceramic interface. The bond strength properties of joints were measured using shear test. The shear strength results demonstrated significant improvement of shear strength of Al/Al₂O₃ joints due to the application of Ti + Nb thin film on alumina substrate. Microstructural investigations of the interface indicated that Al/coating/Al₂O₃ couples have diffusion transition interface which influences the strengthening of these joints. A conclusion could be drawn that the presence of thin film layers changes the character of interaction and leads to the formation of new reaction products in the bonding layer.     

Microstructural characterization of diamond/CBN grains steel braze joint interface using Cu–Sn–Ti active filler alloy

In order to develop the new generation superhard abrasive tools of diamond and cubic boron nitride (CBN), the brazing joint experiments of diamond/CBN crystals and AISI 1045 steel matrix using Cu–Sn–Ti active filler powder alloy were investigated in vacuum furnace. The brazing temperature was 930 °C and the dwelling time was 20 min. Interfacial characteristics of the brazing joint among the diamond/CBN grains, the active filler layer and the steel substrate were analyzed using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction techniques. The results indicated that Ti element in the Cu–Sn–Ti alloys diffused preferentially to the surface of diamond/CBN grits to form a Ti-rich reaction layer in the brazed joints by microanalyses. Moreover, the TiC, TiN and TiB₂ phases in diamond/CBN interface and Cu–Ti phase in steel interface were confirmed by X-ray diffraction phase analysis. The wetting and bonding reactions on diamond/CBN by melting Cu–Sn–Ti alloy were realized through the interfacial reaction products like TiC, TiN and TiB₂ compounds during the brazing process. The adhesive strength experiments of the joint interfaces revealed that the grains were not pulled out from the bond interface. The reliable bonding strength of brazed diamond/CBN grains to the steel substrate can meet the application requirements of high efficiency machining in the industrial field.     

过渡元素改善B4C/Al材料界面润湿性的机理研究

B₄C/Al复合材料是目前最理想的中子吸收材料,但工业上常用的液态搅拌法制备过程中存在着界面润湿性差的问题。本文结合实验及第一性原理的方法,通过研究Al(111)/AlB₂(0001)和Al(111)/TiB₂(0001)界面的结构来分析工业上添加过渡元素Ti对B₄C/Al界面润湿性的改善机制。通过计算发现,Al(111)/TiB₂(0001)界面相对Al(111)/AlB₂(0001)界面具有更高的粘附功值,说明其界面结合更强。进一步对比Ti掺杂二硼化物和AlB₂的偏态密度结构,发现Ti掺杂体具有较低的反键态,表明Ti-3d和B-2p轨道电子杂化后,在B、Ti原子间形成了较强的化学键,从而促进了Al(111)/TiB₂(0001)界面处的强结合作用,提高了Al(111)/TiB₂(0001)界面粘附功,故而改善了B₄C/Al界面的润湿性。根据同样的理论依据,V掺杂体也具有较低的反键态,V和B之间的强结合效果或许能够改善B₄C/Al界面的润湿性,成为又一理想的溶体改性掺杂元素。